Field of the Invention
The present invention relates to coverings for selected structures which, in the path of electromagnetic radiation, would otherwise cause such electromagnetic radiation to be present significantly in regions where unwanted and, more particularly, to coverings for suppressing such electromagnetic radiation in these regions in selected portions of the electromagnetic spectrum to a selected degree.
There are many sources emanating electromagnetic radiation, some by design including transmitters for various communications purposes such as information broadcasts or distance ranging and orientation determinations, and some arising as unavoidable incidents to the operation of various kinds of electrical or electronic equipment. The electromagnetic radiation emanating from these sources is subject to encountering various objects and being reflected therefrom or transmitted therethrough to a greater or lesser degree depending on the material or geometrical characteristics of such objects. Such radiation is particularly well reflected by those objects which are good electrical conductors such as structures of one sort or another containing metals.
In many instances, this reflection or transmission of such electromagnetic radiation places that radiation in regions or locales where it is unwanted. For instance, electromagnetic radiation emanating from a transmitter arrangement in a distance ranging system which impinges on, and is reflected from, any conducting structures in the immediate vicinity thereof will lead to substantial portions of this radiation being reflected directly back to the receiving arrangement of the system with unwanted effects.
On the other hand, a lack of any reflective or absorptive materials about an electromagnetic radiation source can also be a problem if the radiation therefrom is to be confined so as not to interfere with other equipment. Thus, any digital system operating at high switching rates will be a source of electromagnetic radiation which may not be in any way significantly confined if housed in a non-conductive structure such as a plastic enclosure.
Therefore, there is often a need to suppress the occurrence of electromagnetic radiation in certain regions relative to objects such as structures which are in the path of incident electromagnetic radiation from a source of such radiation. Among the means used to accomplish this end has been the use of materials which absorb impinging electromagnetic radiation and convert such radiation to heat to be dissipated within the material to thereby reduce the intensity of the radiation leaving such material. A typical material arrangement for accomplishing this is a composite material having a dielectric host, or binding, material portion in which are dispersed particles, capable of responding to impinging radiation in selected frequency ranges, these together providing an absorbing composite material. Such materials can be provided in various forms including paints or other coatings, sheets to be fastened to an underlying substrate perhaps after being bent to more closely match the substrate shape, preformed material structures to be fastened to a more or less matching underlying substrate, etc.
Various kinds of host or binder dielectric materials have been used including certain polymers and ceramics, among others. Similarly, particles of various natures have been used including metals, magnetic metals, semiconductors, and ferrites.
Such composite materials, however, are difficult to apply or fasten to the underlying substrates. The proper thickness of he material and its mounting are often very important to provide the desired suppression of electromagnetic radiation. The desired characteristics of such coverings can be difficult to achieve uniformly on many kinds of structures with many of these forms of material. These difficulties can be compounded by there often being a need to minimize the number of joinings of such material so that rain or other contaminates cannot reach the underlying substrate, and to reduce installation cost. Further, the desire to have suppression at some frequencies of incident radiation, but not at others, and to a selected degree requires particulates well suited to those selected frequency ranges. Particles are often needed in such quantities to assure reaching the degree of suppression desired that the weight of the absorbing material arrangement becomes too heavy for the system in which it is used.